1. Technical Field
The invention relates generally to the field of gas tube-style laser gyroscopes, and more specifically, but not exclusively, to a system, circuit and method for off-mode-peak operation of Ring Laser Gyroscopes (RLG's) in order to avoid and/or mitigate transverse mode excitation.
2. Description of Related Art
RLG's are used as inertial sensors in guidance, navigation and attitude control applications. For example, RLG's can be used for spacecraft and aircraft guidance, navigation, and attitude control applications. In a typical inertial navigation system application, a pair of RLG's may be mounted on a platform inside a set of gimbals (or mounted in a more prevalent strap-down arrangement commonly used for smaller, lower performance RLG's). Sensors located on the gimbals can detect when the platform rotates. A set of three accelerometers can be attached to the platform to determine in what direction the aircraft or spacecraft is heading, and how the motion of the aircraft or spacecraft is changing in the three directions. Such navigational information can be used, for example, by an aircraft's autopilot to keep the aircraft on course, or by a spacecraft's guidance system to guide the spacecraft into a predetermined orbit.
Essentially, RLG's are used to measure angular rotation rates. For example, in a typical RLG, two laser beams are generated in opposite directions around a closed loop path about the axis of rotation of the RLG. Rotation of the RLG device changes the effective path length for the two beams. This rotation of the device thus produces a frequency difference between the two beams, because the frequency of oscillation of the beams depends on the length of the lasing path. This frequency difference between the beams results in a phase shift between the beams that changes at a rate proportional to the frequency difference. The interaction of the laser beams produces an interference fringe pattern, which moves with a velocity proportional to the rate of angular rotation of the RLG device about the axis. An example of such an RLG device is described in U.S. Pat. No. 6,618,151 to Killpatrick, et al, and Honeywell International Inc. as assignee. A primary advantage of this RLG is that it employs an offset aperture for attenuating undesired modes of laser propagation. Nevertheless, the attenuation of undesired modes of laser propagation is only one of the significant technical problems that exists in the design and manufacture of RLG's.
Another significant technical problem that exists in the design and manufacture of RLG's is in the field of relatively small path length RLG's. For example, an RLG device is typically constructed as a triangular or square glass block cavity filled with helium and neon gas, and integral mirrors at each corner with piezoelectric elements backing at least one mirror so as to allow optical path length adjustments by displacement of that mirror. The performance of such an RLG device is driven primarily by such characteristics as the optical apertures, fill gas elements, discharge current restraints, and path length control of the RLG's, which all play an important role in the establishment of coherent light propagation in the RLG's.
In relatively small path length RLG's, a critical design constraint is that an adequate amount of Path Length Control (PLC) is needed to ensure that an integer number of wavelengths are encountered within the traversed optical cavity. However, the use of smaller sensors in the shorter path length RLG's (e.g., GG1308 RLG's made by Honeywell Inc.) makes the PLC accuracy constraints very challenging. As such, failure to maintain the PLC of short path length RLG's can result in poor performance of the RLG's and, in some cases, a complete lack of operation.
Specifically, the conventional short path length RLG's operate at mode peak and use a Phase-Sensitive Synchronous Demodulator (PSSD) control approach to minimize noise. Essentially, the control loop for the RLG dithers about the mode's peak in an attempt to provide the requisite PSSD control. However, the modes (humps) observed in the short path length RLG's are not perfectly shaped. In other words, the short path length RLG's have design issues that can deviate significantly from those RLG's operating with a perfectly (or nearly perfectly) shaped hump. Consequently, if the PLC is dithering about the mode's peak in a conventional short path length RLG, any amount of additional noise experienced can cause the PLC to wander into an area (e.g., valley) with one or more undesirable transverse modes.
Therefore, it would be advantageous to have a solution to this existing problem of maintaining the PLC of short path length RLG's, which identifies a suitable PLC operating point that can allow relaxation of the relatively tight PLC requirements while maintaining optimum performance of the RLG's. As described in detail below, the present invention provides such a solution.